This invention relates to the protection of sensitive electronics from voltages exceeding a rated voltage (over voltages). Such over voltages typically occur as a result of lightning strikes.
During aircraft operation it is not uncommon for the aircraft to get struck by lightning, say while flying through a storm. As such, in the design of aircraft, it is necessary to create protection against lightning strikes. This protection is extended not only to persons on board the plane, but also to voltage sensitive electronics for operating the plane. Examples of sensitive electronics may include fuel sensors, altitude sensors, wing controls, etc. If lightning were to strike and these electronics had no protection, the sensitive components (sensing resistors, capacitors, etc.) could be destroyed or have their sensing capabilities damaged. This could lead to malfunctioning sensors and airplane controls.
Another situation where other electronics are subjected to a similar over voltage occurs when there is a large power spike across a power distribution network. In instances such as a power spike unprotected electronic equipment is often damaged or destroyed. As such, the described protection system would be desirable in those applications as well as in an aircraft.
In the prior art one of two methods has typically been used to protect sensitive electronics from over voltages. The first method involves using “voltage clamping” devices placed in parallel with the protected electronics. A voltage clamping device works by allowing any voltage above a rated value to run off to ground. These devices provide protection by converting excess voltage into current, and running the current through a circuit, typically a resistor bank, that converts the current into heat energy, which is then radiated off. This regulates (or clamps) the voltage across the parallel load to a certain predetermined level.
This method has three main drawbacks. The first drawback is that voltage clamping circuits typically convert the current into heat very rapidly, which necessitates an increase in the size, weight, and expense of the device to compensate for this rapid heating. The second drawback resulting from this method is the lack of reliability. The reliability of the protection may become in question due to repeated occurrences of over voltage. If the over voltage event occurs repeatedly it can put wear on the resistor bank and the clamping circuit, thus reducing the resistor bank's and the clamping circuit's effectiveness. The third drawback is the lack of testability of the protection circuit. With this method of protection there is no reliable way to test the protection circuit without subjecting the circuit to a potentially destructive application of voltage. A test could determine the functionality of the protective circuit at the time of the test. However, there would be no indication as to whether the test itself damaged the protective capabilities. Additionally, if the protection circuit were not functional the test itself could destroy the protected electronics.
The second method commonly used to provide over voltage protection involves using a series switch technique to disconnect the circuit when a sensor detects an incoming over voltage. A series switch involves the placement of a switch between the source of the over voltage and the load (electrically the switch is in series with the load). It achieves its function by first sensing the incoming voltage, then using a controller to toggle the switch controlling the load, thereby opening the circuit and preventing any voltage from flowing through the load. The major drawback of this method is that the power supply to the electronics is disrupted, thus necessitating the use of some other means of providing continuous power, such as a battery back-up or other power storage device. Another drawback associated with this method is the reliability and accuracy of switching circuits under varying conditions.